Apple - Can there be two separate Time Machine Backups on the same external drive?

You can use an external hard disk formatted with HFS+ as a Time Machine target for several Macs without problems. Time Machine itself will make sure that the data is not getting mixed up by using the name of each Mac as part of the path.

If you make two different partitions in the external HD, you can easily have the two different Time Machine backup in the each partition.

You only need to specify the partition to use for the MacBook during the initial Time Machine setup. Then the Mac would know which partition to use for its backup the next time you connect the external HD. The same goes for the second MacBook.

By doing so you could limit the amount of space taken up by he Time Machines of both Macs.